Vehicle bodies or components thereof are coated during a series of steps constituting an overall coating process, while the vehicle bodies or components thereof are conveyed along a coating line. In one scenario, a "body frame integral" (BFI) constitutes the car welded together in a complete body which then is advanced on a carrier through the coating line. This is the traditional method.
In another scenario known as "space-frame construction" or "panels off", the exterior components of the vehicle body, namely hood, roof, deck lid, fenders, doors and quarter panels, are coated prior to assembly. These components (panels) are carried on a carriage in their in-body position (the position they will assume after assembly), and taken through the entire coating line in the unassembled in-body position.
The steps conducted in the coating line may involve applying a corrosion protective electrodeposited layer, a primer, an intermediate basecoat comprising pigment with or without flakes or mica, and finally applying a top coat or overcoat, sometimes referred to as a "clear coat", to provide a mirror-like, high gloss finish. Two basic types of basecoats are used today in combination with a clear coat. A basecoat may be water-borne or organic solvent-borne. Typically, a two component isocyanate clear coat or one component melamine composition is applied over the basecoat. The primer, basecoat and clear coat are referred to as "paints" and are applied in a spraying step, each followed by a drying step. The drying step may have two parts: a setting step and a baking step. The setting step is designed to volatilize a solvent typically when a water-based paint is used. The baking step bakes the applied paint at an elevated temperature. In instances where an organic solvent-based paint is used, the drying step usually involves the baking step only, because such solvents are more volatile than water.
All baking is conducted in two basic stages: a first stage to set the applied coating sufficiently to avoid inclusion of or adhesion of dust, and a second stage to cure the coating. The first stage is usually in an infrared oven, and the second stage is usually in a convective baking oven (hot air).
When painting vertical components of the vehicle body, the paint is generally applied with a spray gun from a direction transverse to the surface of the vertical component. Since the surface of the component to be painted is oriented generally vertically, the paint may droop and sag due to gravity so that the thickness of the coating or film formed when the paint is ultimately dried is uneven. Some sagging may occur by influence of gravity when the paint is applied. However, such sagging is more particularly a problem when baking the applied paint. One approach used to overcome this problem is to include rheology control components in the paint, and particularly the top coat, so that the influence of gravity is lessened. Such rheology components basically cause the paint to be thicker, more adhesive to the vertical panels, to have higher viscosity (decreased fluidity), and to have a tendency to level or flow on the panels.
Another alternative for attempting to overcome problems with gravity and paint sagging is as disclosed in U.S. Pat. No. 4,874,639 issued to Matsui et al in 1989, wherein paint is sprayed onto an assembled vehicle body (BFI), and then the entire body is rotated about its horizontal axis until the paint sprayed thereon is set and baked. Such a system requires significant alteration of existing coating lines, spray paint booths and baking ovens in order to accommodate the clearances required to rotate an entire body assembly. In addition, significant mechanical energy is needed to rotate an entire vehicle body having a weight on the order of 1000 pounds to 2000 pounds.
The degree of evenness of the coated surface is a standard which is used to evaluate the quality of the coating. Irregularities in the coated surface, sometimes referred to as "orange peel", may occur. Such irregularities are measured in units called "tension". A rating of 20 tension is equivalent to polished black glass, so that if the surface of a car had a coating which achieved a 20 tension, it would be equivalent to looking in a mirror with no distortion or orange peel.
The rotational bake system presently used includes a double clear coat, where the first clear coat application is sanded to smoothness and then a second clear coat is applied. With the rotational bake system presently in use, it is reported that ratings of up to about 19 tension are achievable, but this rotational bake system requires that a double clear coat be applied while rotating the entire car at significant cost and with total reconstruction of the coating line to accommodate such rotation.
Therefore, what is need is an improved method and apparatus for the coating line.